banbh banbh - 3 years ago 58
R Question

Does geom_abline(...) plot data multiple times?

Does using

(as well as
) result in multiple overplotting of the same line when used "naively"?

For example, say we are interested in the following faceted scatterplot:


k <- 4
data.frame(id = letters[1:k],
m = rnorm(k),
b = rnorm(k))[rep(1:k, 30),] %>%
mutate(x = rnorm(n()),
eps = 0.1*rnorm(n()),
y = m*x + b + eps) %>%
ggplot(aes(x, y)) +
geom_point() +
facet_wrap(~ id) ->

The easiest way to add an ab-line to it is as follows:

print(p + geom_abline(aes(slope = m, intercept = b), color = 'red'))

Is that the "right way" to do it? In particular, doesn't the above plot the ab-line 30 times in each facet? For example this makes it seem like it does:

print(p + geom_abline(aes(slope = m, intercept = b + eps), color = 'red'))

If so, is it better to do something like this?

print(p + geom_abline(aes(slope = m, intercept = b),
data = Z %>% group_by(id) %>% summarize(m = unique(m), b = unique(b)),
color = 'red'))

Note that this produces something visually indistiguishable from the first plot. My question is about the right way to use these ggplot functions.

Answer Source

geom_abline takes care already of multiple lines at the same spot by making the values unique. You can also verify that by running e.g. p + geom_abline(aes(slope = m, intercept = b), color = 'red', alpha = .1) - if it were 30 lines at the same spot, they would be opaque.

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